Bidirectional multichannel optical telecommunication system

ABSTRACT

An optical amplifier having a first input for inputting first optical signals propagating in a first direction having a first series of mutually distinct wavelengths; a first output for outputting second optical signals propagating in a first direction having the first series of mutually distinct wavelengths; a first amplifier for amplifying the first optical signals; a first optical circulator having an input port coupled to the first amplifier, an intermediate port coupled to a first end of a first selective reflection circuit, and an output port coupled to the first output; and a second optical circulator having an intermediate port coupled to a second end of the first reflection circuit, an output port coupled to a first selective splitter, and an input port coupled to a second selective splitter, where the input and output ports have passbands centered around a first predetermined wavelength of the first series.

DESCRIPTION

It is an object of the present invention to provide a bidirectionalmultichannel telecommunication system, a bidirectional opticalamplifier, and a method for the bidirectional transmission of opticalsignals.

In the latest telecommunication technology, it is known to use opticalfibers to send optical signals carrying information for long-distancecommunication.

It is also known that optical signals sent in an optical fiber undergoattenuation along the way, making it necessary to amplify the signal sothat it will travel the entire required distance and reach the receivingstation at a power level sufficient for correct signal reception.

Said amplification may be effected by means of appropriate amplifiersplaced at predetermined intervals along the line, which periodicallyboost the power of the transmitted optical signal.

Optical amplifiers are suitably used for this purpose, by which thesignal is amplified while remaining in optical form, i.e. without theoptoelectronic detection and electrooptical regeneration of same.

Said optical amplifiers are based on the properties of a fluorescentdopant (e.g. erbium) which, if appropriately excited by the applicationof luminous energy, provides a strong emission in the wavelength bandcorresponding to the minimum attenuation of light in silica-basedoptical fibers.

Said amplifiers are unidirectional devices, i.e. having a predetermineddirection of travel of the optical signal inside them.

This is due, as described for example in U.S. Pat. Nos. 5,204,923 and5,210,808 of the Applicant, to the fact that the optical amplifiers,particularly if high gain values are required, incorporateunidirectional components that prevent signals reflected outside theamplifiers, e.g. due to Rayleigh scattering along the optical linefibers connected to the amplifiers, from returning into the amplifier,causing interferometric noise.

For the bidirectional transmission of optical signals, known technologygenerally calls for the use of two separate communication lines,equipped with their respective amplifiers, each of which is used tocommunicate in a single direction. This results in a high connectioncost.

Nevertheless, some technologies are know whose objective is to permitbidirectional transmission on fiber-optic lines by means ofbidirectional optical amplifiers.

Bidirectional amplification schemes have been presented with the use ofa single unidirectional amplifier that exploit the possibility offluorescent doped amplifiers to independently amplify signals atdifferent wavelengths.

A bidirectional amplifier based on this principal is described in thearticle by S. Seikai et al.: “Novel Optical Circuit Suitable forWavelength Division Bidirectional Optical amplification”, published inElectronics Letters, vol. 29, no. 14, Jul. 8, 1993, pages 1268-1270. Itdiscusses a device placed along a fiber-optic transmission line in whichtwo signals with different wavelengths propagate in opposite directionsand which consists of wavelength selective couplers and a known type ofunidirectional doped-fiber amplification unit connected by sections ofpassive optical fiber. The wavelengths of the signals are both internalto amplification band of the doped fiber. By means of selective couplersthe two signals at different;wavelengths are input to different opticalpaths. The two optical paths coincide only in the section correspondingto the amplifying fiber, which is passed through by the two signals inthe same direction. The device has a problem of instability caused byinternal reflections at a wavelength between those of the propagatingsignals, a problem resolved through the addition of filters, some ofthem adjustable, which results in a highly complicated structure and theneed to use devices to accurately and continuous adjust said filters.

Patent application EP96100586, filed on Jan. 17, 1996 in the name of theApplicant, describes, among other things, a bidirectional opticalamplifier comprising:

an optical amplification unit including at least an optical isolator,

two optical input and output ports for at least two optical signalshaving opposite propagation directions, said signals having,respectively, a first and a second distinct wavelengths,

two first and two second wavelength selective optical couplers, havingrespectively a first wavelength passband, including said firstwavelength, and a second wavelength passband, including said secondwavelength, with no overlapping,

said amplification unit being connected between two opposite nodes of anoptical bridge circuit, to whose other opposite nodes said input andoutput ports are connected, and said first and second selective opticalcouplers being present at the nodes of said bridge circuit, in whichsaid first and second selective couplers are arranged symmetrically withrespect to the amplification unit and with respect to the input andoutput ports of said optical signals.

Bidirectional amplification schemes have also been presented withseparation of the counterpropagating signals and the use of aunidirectional amplifier for each direction.

As an example, the article by C. W. Barnard et al. “Bidirectional FiberAmplifiers” , published in IEEE Photonics Technology Letters, vol. 4,no. 8, August 1992, pages 911-913, describes bidirectional erbium-dopedfiber amplifier repeaters for bidirectional fiber networks and OTDRfault detection. At each repeater the counterpropagating signals areseparated, amplified separately, then recombined. Signal separation isdone by a bidirectional fiber coupler or an optical circulator.According to the authors, for example, one propagation direction couldbe assigned 1525-1535 nm, the other could be assigned 1550-1560 nm, andthe OTDR wavelength could be 1548 nm.

Optical telecommunication systems are known with wavelength divisionmultiplexing (WDM) transmission. In these systems, a number of channelsare sent, i.e. a number of mutually independent transmission signals, onthe same line, usually consisting of an optical fiber, by means ofoptical wavelength multiplexing. The transmitted channels can be eitherdigital or analog and are mutually distinguished because each of them isassociated with a specific wavelength.

U.S. Pat. No. 5,283,686, in the name of D. R. Huber, describes, amongothers, optical systems including an optical amplifier and anarrow-bandwidth optical filter for removing undesired spontaneousemission. An in-fiber Bragg grating reflector reflects substantiallyonly the input amplified signal back to an optical circulator port. Theundesired emission exits from the grating reflector and is removed fromthe system. Cascaded grating reflectors are used in a wavelengthdivision multiplexing (WDM) system. The article of K. Y. Chen et al.,“Demonstration of in service supervisory repeaterless bi-directionalwavelength division multiplexing transmission system”, vol. 7, no. 9,Sep. 1, 1995, states that repeaterless long distance fiber transmissionsystems using erbium doped fiber amplifiers as a transmitter poweramplifier and/or as a receiver preamplifier have many applications, inwhich cases it is infeasible or impossible to have an in line amplifier,such as island hopping and intracity links. In this letter, an inservice supervisory repeaterless bi-directional six WDM channeltransmission over a 200 Km single fiber link is demonstrated.

The patent application EP 0 535 590 discloses a two way repeaterapparatus for directly amplifying optical signals, to be transmitted inmutually opposite directions. Said repeater receives an outward inputoptical signal S1 of 1.552 microns in wavelength at an input/outputterminal 1, and receives an inward optical signal S1r of 1.536 micronsin wavelength at an input/output terminal 20.

The patent application DE 36 32 047 A discloses a communication systemused for digital narrow band and wideband signals transmitted in bothdirections via a single optical waveguide.

The article of M. J. Chawki et al., “Evaluation of an optical boostedadd/drop multiplexer OBADM including circulators and fiber gratingfilters”, proceeding of ECOC, vol. 1, Sep. 17, 1995, discloses two OBADMconfigurations wherein bi-directional EDFA and fiber grating filters areplaced between the 2 circulators.

The patent application EP 0 729 248, corresponding to the patent U.S.Pat. No. 5,633,741, discloses that WDM optical fiber communicationsentails bi-directional transmission with at least two WDM channels inopposite transmission directions in a single fiber. Communication is bya single fiber transmission line served by bi-directional amplifiers.The amplifier includes fiber gratings that serve as filters andstabilize against oscillation due to reflections and to Rayleigh backscattering.

It is known that the wavelength bandwidth available for signals, inoptical communication systems with doped-fiber amplifiers, is limited bythe characteristics of the active dopant used. In the case of amplifiersdoped with erbium, for example, said bandwidth is limited to thewavelength bandwidth included approximately between 1530 and 1565 nm.

The Applicant has observed that the known WDM communication systems arefurther limited regarding the number of channels, i.e. the independentwavelengths usable for transmission within said amplification bandwidth.

The need to reduce noise, particularly of the interferometric type,associated with the retroreflection of signals or noise toward theamplifiers, makes it imperative to maintain a good isolation between thechannels at the various wavelengths propagating in the system, i.e. agood suppression of radiation at intermediate wavelengths between thoseof the communication channels. This isolation may be obtained, in knownsystems, only by maintaining a channel separation greater than apredetermined minimum value, which limits the number of channels usablein the available wavelength bandwidth. This minimum value depends on thecharacteristics of the components employed in the system, such as thespectral characteristics of the wavelength selective components (e.g.bandwidth, center-band attenuation, figure of merit) and wavelengthstability (thermal and temporal) of the filters and of the opticalsignal sources.

Furthermore, to separate signals with different wavelengths, e.g. todrop some of them to receivers placed in intermediate amplificationnodes or to send them, at the receiving station, to separate receivers,contiguous channels (in terms of wavelength) must be separated by morethan a predetermined limit value.

Said limit value depends primarily on the characteristics of thewavelength selective components employed along the optical signal path.

By means of the present invention it is possible to transmit in anoptical telecommunication system a number of independent opticalchannels greater than is permitted by known techniques, by employingwavelength selective components of equal characteristics.

The Applicant has found that by feeding wavelength-contiguous channelsin two opposite directions in the system it is possible to usefiltering, multiplexing and demultiplexing means having spectralresolution greater than the spacing between the channels.

According to a first aspect, the present invention concerns abidirectional multichannel optical telecommunication system comprising:

means for generating optical signals suitable for generating at leastthree optical signals having, respectively, a first, a second and athird mutually distinct wavelengths;

a line for transmitting optical signals;

means for inputting said first and third optical signals to saidtransmission line, placed at a first end of said transmission line;

means for inputting said second optical signal to said transmissionline, placed at a second end of said transmission line opposite fromsaid first end; characterized by the fact that the value of said secondwavelength is comprised between said first and third wavelengths.

According to another aspect, the present invention regards amultichannel optical telecommunication system for transmitting opticalsignals comprising:

a fiber-optic line having a first and a second end;

means for inputting, at said first end of said line, first opticalsignals propagating in a first direction and having a first series ofmutually distinct wavelengths;

means for inputting, at said second end of said line, second opticalsignals propagating in a second direction, opposite to said firstdirection, and having a second series of wavelengths mutually distinctand distinct from the wavelengths of said first optical signals;

first means for demultiplexing said optical signals, at said second endof said line, and second means for demultiplexing optical signals atsaid first end of said line, said first and said second means ofdemultiplexing being suitable for demultiplexing signals distant fromeach other in wavelength by a quantity greater than or equal to apredetermined minimum value.

Characterized by the fact that the wavelengths of said first signalsdiffer from each other by a quantity greater than or equal to saidminimum value, the wavelengths of said second signals differ from eachother by a quantity greater than or equal to said minimum value, whilethe wavelengths of said first signals differ from the wavelengths ofsaid second signals by a quantity greater than or equal to half saidminimum value.

In one of its versions, said system comprises bidirectionalamplification means optically connected along said fiber-optic line.Preferably, said means comprise:

means for separating said first signals from said second signals;

means for amplifying said first signals;

means for amplifying said second signals;

means for combining said first and second signals.

Said means for separating and said means for combining includerespective optical circulators.

Said means for amplifying said first and second signals may beunidirectional and may comprise respective comb filters suitable fortransmitting signals at wavelengths within bands including thewavelengths of said first and said second signals, respectively, andsuitable to attenuate radiation at wavelengths external to said bands.Said comb filters may comprise an optical circulator and Bragg gratingreflectors cascaded at an intermediate port of said circulator.

In one version of said system, said minimum distance value is less thanor equal to 1 nm.

According to a third aspect, the present invention regards amultichannel optical telecommunication system for the transmission ofoptical signals comprising:

a fiber-optic line;

multiplexing and demultiplexing means, for adding and dropping, in atleast two different positions along said line, optical signals havingmutually distinct wavelengths, said multiplexing and demultiplexingmeans having a spectral resolution greater than the minimum spacingbetween channels, characterized by the fact that channels contiguous inwavelength are fed in opposite directions along said line.

According to a fourth aspect, the present invention regards abidirectional optical amplifier comprising:

means for inputting first optical signals propagating in a firstdirection and having a first series of mutually distinct wavelengths;

means for inputting second optical signals propagating in a seconddirection, opposite said first direction, and having a second series ofwavelengths, mutually distinct and distinct from the wavelengths of saidfirst optical signals;

means for separating said first optical signals from said second opticalsignals;

means for amplifying said first signals;

means for amplifying said second signals;

means for combining said first and said second signals; characterized bythe fact that the wavelengths of said first and said second signals aremutually staggered.

Said means for separating and said means for combining advantageouslycomprise respective optical circulators.

Said means for amplifying said first and second signals may beunidirectional and may includes respective comb filters suitable fortransmitting signals at wavelengths within bands comprising therespective wavelengths of said first and said second signals andsuitable for attenuating radiation at wavelengths external to saidbands. Said comb filters may comprise an optical circulator and Bragggrating reflectors cascaded at an intermediate port of said circulator.

According to a fifth aspect, the present invention regards a method forthe bidirectional transmission of optical signals along an opticalcommunication line comprising the steps of:

generating first optical signals at a first series of wavelengths;

generating second signals at a second series of wavelengths, differentfrom the wavelengths of said first series;

transmitting said first signals in a first direction along the opticaltelecommunication line;

transmitting said second signals in a second direction along the opticaltelecommunication line; in which the wavelengths of said first signalsare staggered with respect to the wavelengths of said second signals.

In one version, said method comprises the step of amplifying said firstand said second signals along the optical communication line, which inturn preferably comprises the steps of;

separating said first from said second signals;

amplifying said first signals by means of a first optical amplifier;

amplifying said second signals by means of a second optical amplifier;

recombining said first and said second signals along saidtelecommunication line.

Additional information may be derived from the following description,with reference to the attached drawings showing:

in FIG. 1 diagram of an optical telecommunication system;

in FIG. 2 diagram of a transmission interfacing unit;

in FIG. 3 diagram of an optical power amplifier;

in FIG. 4 diagram of an optical preamplifier;

in FIG. 5A diagram of an optical demultiplexer;

in FIG. 5B diagram of a wavelength-selective optical splitter;

in FIG. 6 diagram of a bidirectional optical amplifier

in FIG. 7 diagram of an optical amplifier associated with a device of afirst type for adding and dropping signals;

in FIG. 8 diagram of an optical amplifier associated with a device of asecond type for adding and dropping signals.

As shown in FIG. 1, a bidirectional optical telecommunication systemwith wavelength-division multiplexing, according to the presentinvention, comprises two terminal stations A and B, each of whichincludes a respective transmission station 1A, 1B and a respectivereceiving station 2A, 2B.

In particular, in the version shown in the figure, transmission station1A comprises 16 optical signal transmitters with a first series ofwavelengths, indicated with odd-numbered subscripts, λ₁, λ₃, . . . , λ₃₁(included, for example, in the wavelength band of 1530-1565 nm) andtransmission station 1B comprises 16 optical transmitters with a secondseries of wavelengths, indicated with even-numbered subscripts, λ₂, λ₄,. . . , λ₃₂.

The wavelengths of the second series are selected so that they arestaggered with respect to the wavelengths in the first series.

In other words, each pair of wavelengths of one series encompasses awavelength of the other series.

In the present case, the wavelengths of the two series will be indicatedas staggered, more generally, even when the wavelengths of the signalsof each of said series, corresponding to optical signals emitted by oneof the transmission stations 1A, 1B and propagating in the system in oneof the two directions, are separated (in frequency) by a quantitygreater than or equal to 2D, where D indicates the minimum bandwidth (infrequency) of the wavelength selective components used in the system toseparate the signals at the various wavelengths.

The number of independent wavelengths used for the signals for eachtransmission station is not limited to the value of 16 indicated in thedevice described and may assume a different value. The number ofwavelengths, corresponding to the number of optical channels used fortransmission in each direction, may be selected in relation to thecharacteristics of the telecommunication system. In particular, in atelecommunication system according to the present invention, it ispossible, after the system implementation, to increase the number ofchannels to increase the transmitting capacity of the system, e.g. toaccommodate an increased traffic demand, as will be indicated below.

The wavelengths may be selected so that the corresponding frequenciesare equally spaced within the available spectral amplification band, soas to utilize said band efficiently.

It is possible, however, for the frequencies to be totally or partiallyunequally spaced, e.g. so as to reduce the effect of non-linearphenomena, such as four wave mixing (FWM) in optical fibers used fortransmitting the signals.

The useful amplification band of the amplifiers may also be constitutedof two or more distinct spectral bands separated by spectral bands notwell-suited for signal transmission or amplification, e.g. due to theparticular spectral characteristics of the amplifiers or optical fibersemployed in the telecommunication system. In this case, the wavelengthsof the communication channels may, for example, be selected such thatthe corresponding frequencies are equally spaced within each individualspectral band, with the separation between adjacent channels propagatingin the same direction greater than or equal (in frequency) to twice saidvalue D.

As an example, the wavelengths may assume values between about 1535 nmand about 1561 nm, where consecutive wavelengths, in ascending order,are used alternately for each of the two series λ₁, λ₃, . . . , λ₃₁ andλ₂, λ₄, . . . , λ₃₂. The spacing between the 32 total wavelengths, inthis case, is about 0.8 nm.

The optical transmitters comprised in transmission stations 1A and 1Bare modulated, directly or with external modulation, according to systemrequirements, in particular in relation with the chromatic dispersion ofthe optical fibers in the system, with their lengths, and with theintended transmission velocity.

The outputs of each transmitter of transmission stations 1A and 1B areconnected to multiplexers 3A and 3B, respectively, which combine theiroptical signals each toward a single output, connected respectively tothe input of optical power amplifiers 5A and 5B. The outputs of theseamplifiers are connected to an input port of optical circulators 7A and7B.

An intermediate port of optical circulators 7A and 7B is connected toone end of an optical line 8, which connects the two terminal stations Aand B together.

The optical fiber of optical line 8 is normally a singlemode opticalfiber of the step index or dispersion shifted type, convenientlyincluded in a suitable optical cable, and has tens (or hundreds) ofkilometers of length between each amplifier, up to the desiredconnection distance.

Inserted along line 8 are bidirectional optical amplifiers 9. Each ofthem comprises two optical circulators 91 and 92 and two opticalamplifiers 93 and 94, which will be described further on. A central portof each optical circulator is connected to the optical fiber of line 8,e.g. through an optical connector, and acts as an input/output port forthe bidirectional amplifier. Optical amplifier 93 is optically connectedbetween an output port of optical circulator 91 and an input port ofoptical circulator 92. Optical amplifier 94 is optically connectedbetween an output port of optical circulator 92 and an input port ofoptical circulator 91.

Although FIG. 1 indicates two bidirectional optical amplifiers 9, theremay be one or more bidirectional optical amplifiers in succession,depending on the overall length of the optical connection and the powerin the various sections of it. A fiber section between a terminalstation and an amplifier, for example, or between two successiveamplifiers, may be on the order of 100 kilometers long.

Receiving stations 2A and 2B are connected to the output ports ofoptical circulators 7A and 7B through preamplifiers 6A and 6B anddemultiplexers 4A and 4B.

The optical circulators are passive optical components, commonlyequipped with three or four access ports placed in an ordered sequence.After defining a first arbitrarily chosen access port as “input port”,the next ports in sequence will be indicated as central port and outputport The optical circulators transmit unidirectionally the radiationinput by each of the ports to one only of the other ports, namely thenext one in sequence. The circulators used in the present invention arepreferably of the polarization-independent type.

Preamplifier, in the context of the present invention, is an amplifierdimensioned to compensate the losses of the last section of optical lineand the insertion losses of demultiplexer 4A or 4B, so that the powerlevel of the signal input to the receiver is suited to the sensitivityof the device. It is also the task of the preamplifier to limit signaldynamics, reducing the power level variations of the signals at thereceiver input with respect to the power level variation of the signalsfrom the transmission line. Demultiplexers 4A and 4B are suited fortaking 16 signals overlapped in a single input port connected to theoutput of preamplifier 6A, 6B and separating them on to 16 opticalfibers, in accordance with their respective wavelengths.

When the optical signals for transmission are generated by signalsources with their own transmission characteristics (such as wavelength,modulation type, power) different from those envisaged for the describedlink, each transmission station 1A, 1B comprises interfacing units 901,903, . . . , 931 and 902, 904, . . . , 932, respectively, for receivingthe optical signals generated by transmission stations 1A, 1B, detectingthem, regenerating them with new characteristics suited to thetransmission system and sending them to multiplexers 3A, 3B.

In particular, said interfacing units generate optical working signalswith wavelengths λ₁, λ₃, . . . λ₃₁ and λ₂, λ₄, . . . , λ₃₂,respectively, suited to the system requirements as described below.

U.S. Pat. No. 5,267,073 by this same Applicant, describes interfacingunits comprising in particular a transmission adaptor for converting anoptical input signal into a form well-suited for the opticaltransmission line and a reception adaptor for converting the transmittedsignal into a form well-suited for a reception unit.

For use in the system of the present invention, the transmission adaptorcomprises, preferably, an externally modulated laser as an output signalgeneration source.

The diagram of a transmission interfacing unit 900, of the typewell-suited for use within the context of this invention, is shown inFIG. 2 in which, for the sake of clarity, the optical connections arerepresented by solid lines, while the electrical connections arerepresented by broken lines.

The optical signal, coming from an external source 207, is received by aphotodetector (photodiode) 208, which emits an electrical signal whichis fed to an electronic amplifier 209.

The electrical signal output by amplifier 209 is fed to a circuit 210that drives a modulable laser emitter, designated overall as 211, thatgenerates an optical signal at the selected wavelength, containing theinformation of the incoming signal.

If appropriate, a circuit 212 for inputting a service channel may beconnected to driving circuit 210.

Modulable laser emitter 211 includes a continuous emission laser 213 andan external modulator 214, e.g. of the Mach-Zehnder type, driven by theoutput signal of circuit 210.

A circuit 215 controls the emission wavelength of laser 213, keeping itconstant at the specified value and compensating for any externaldisturbances such as temperature and the like.

Transmission interfacing units of the type indicated are described inthe aforesaid patent and marketed by the Applicant under the designationTXT/EM-XXX.

As an alternative, the laser transmitters in transmission stations 1Aand 1B may be laser transmitters operating at the selected wavelengths,e.g. using DFB lasers at wavelengths λ₁, λ₃, . . . , λ₃₁ and λ₂, λ₄, . .. , λ₃₂, respectively.

Preferably, the wavelength of each source used for the signals is stablewithin +/−0.25 nm, more preferably within +/−0.1 nm.

With reference to FIG. 1, the optical circulators are componentsavailable commercially. A model well-suited for use in the presentinvention, for example, is the PIFC-100 produced by E-TEK DYNAMICS Inc.,1885 Lundy Ave., San Jose, Calif. (USA), characterized by an attenuationof 0.7 dB in transmission between two consecutive ports and by aresponse substantially independent from polarization.

Power amplifiers 5A and 5B raise the level of the signals generated bytransmission stations 1A and 1B to a value sufficient to travel thesection of optical fiber separating them from the receiving station oramplification means with sufficient terminal power to ensure therequired transmission quality.

A power amplifier well-suited for use in the present invention will nowbe described with reference to FIG. 3.

The power amplifier represented is of the two-stage type. A firstamplification stage comprises an active fiber 32, pumpedcounterdirectionally by a pumping source 34 through a dichroic coupler33.

A second amplification stage comprises an active fiber 36, pumpedcounterdirectionally by a pumping source 38 through a dichroic coupler37.

An amplifier input 310 is connected through a first optical isolator 31to the first amplification stage, and precisely to active fiber 32,whose output terminates in a branch of dichroic coupler 33. Pumpingsource 34 is connected to a second branch of dichroic coupler 33, whilea third branch of the same dichroic coupler constitutes the signaloutput of the first stage.

A second optical isolator 35 is located between the output of the firststage and an input of active fiber 36 of the second stage, whose outputterminates in a branch of dichroic coupler 37. Pumping source 38 isconnected to a second branch of dichroic coupler 37, while a thirdbranch of the same coupler constitutes the signal output of the secondstage, which terminates in an output 320, consisting preferably of avery-low-reflection optical connector, e.g. an angled connector withreflectivity less than −55 dB. Optical connectors of this type aremarketed, for example, by SEIKOH GIKEN, 296-1 Matsuhidai, Matsudo, Chiba(Japan).

Output 320 is connected, in the telecommunication system of FIG. 1, withan optical circulator (7A or 7B). This circulator permits theunidirectional passage of radiation output by the power amplifier andprevents radiation from entering by that output. The circulator is thusequivalent to an additional optical isolator connected to the amplifieroutput, particularly in limiting its interferential noise.

Active optical fibers 32 and 36 are preferably silica optical fibers. Arare earth is used as a dopant, preferably erbium. Aluminum, germaniumand lanthanum, or aluminum and germanium, may be advantageously used assecondary doping agents.

The concentration of dopants may correspond, for example, to anattenuation of around 7 dB/m, for the active fiber in the absence ofpumping.

In a preferred embodiment, the amplifier described uses erbium-dopedactive fibers of the type presented in detail in patent application EP677902, in the name of the Applicant.

The lengths of active fibers 32 and 36 may be around 7 m and 5 m,respectively.

For dichroic couplers 33 and 37, fused-fiber couplers may be used,formed of monomodal fibers at 980 nm and in the 1530-1565 nm wavelengthband, with optical power output variation with respect to polarization<0.2 dB.

Dichroic couplers of the type indicated are known and commercial and areproduced, for example, by the aforesaid E-TEK DYNAMICS.

Optical isolators 31 and 35 are of the type independent of thetransmission signal polarization, with isolation greater than 35 dB andreflectivity less than −50 dB. The isolators are, for example, model MDL1-15 PIPT-A S/N 1016 of the firm ISOWAVE, 64 Harding Ave., Dover, N.,J.(USA) or model PIFI 1550 IP02 of the aforesaid E-TEK DYNAMICS.

Pumping sources 34 and 38 may be, for example, quantum well lasers withan emission wavelength of λ_(p)=980 nm. The optical emission powerenvisaged is around 75 mW for source 34 and 90 mW for source 38.

Lasers of the type indicated are produced, for example, by LASERTRONINC. 37 North Avenue, Burlington, Mass. (USA).

A power amplifier like the one described furnishes, for example, outputpower of around 16 dBm, with a noise figure of around 5 dB.

The power amplifier described with reference to FIG. 3 usescounterpropagating pumping for both amplification stages.Counterpropagating pumping for both stages or for just one of them, thefirst stage in particular, are equally possible. The choice of whichconfiguration to use is left to the skilled in the art, according to thecharacteristics of the overall communication system.

The optical power amplifier may also be embodied as a single-stageamplifier, depending on the gain required and the characteristics of thetelecommunication system in which it is to be used. It is possible, forexample, with reference to the device in FIG. 3, to omit active fiber36, dichroic coupler 37 and pumping source 38. This simplerconfiguration offers less optical output power and may be sufficient forparticular embodiments of the amplification system, e.g. with a smallernumber of communication channels or with optical fiber sections oflimited length downstream of the amplifier.

Preamplifiers 6A and 6B of the system in FIG. 1 are, for example,optical amplifiers of the type that will be described now with referenceto FIG. 4, which represents a two-stage preamplifier.

A first amplification stage comprises a first active fiber 44, pumped bya pumping source 42 through a dichroic coupler 43, a differentialattenuator 45, connected to the output of active fiber 44, to attenuatethe telecommunication signals without significantly attenuating theresidual pumping radiation, and a second active fiber 46 pumped by meansof said residual pumping radiation.

A second amplification stage includes an active fiber 47, pumped by apumping source 49 through a dichroic coupler 48.

An input 410 of the preamplifier, consisting preferably of avery-low-reflection optical connector, e.g. of the type previouslyindicated, is connected to the first amplification stage, and preciselyto a first input of dichroic coupler 43, to a second input of whichpumping source 42 is connected. An output of dichroic coupler 43terminates in active fiber 44.

Input 410 is connected, in the telecommunication system in FIG. 1, to anoptical circulator (7A or 7B). This circulator permits theunidirectional passage of radiation to the preamplifier and preventsradiation from exiting that input. The circulator is thus equivalent toan additional optical isolator connected to the amplifier input,particularly in limiting interferential noise.

Differential attenuator 45 is connected between active fiber 44 andactive fiber 45. Its function is to attenuate the telecommunicationsignals by a predetermined quantity without significantly attenuatingthe residual pumping radiation from active fiber 44. A differentialattenuation of the signals with respect to the pump, in a suitableintermediate position between two sections of active fiber of an opticalamplifier, as described in patent applications EP567941 and EP695050 inthe name of the Applicant, makes it possible to compress the amplifierdynamics, i.e. to limit the power variations of the signals output bythe amplifier with respect to the power variations of the input signals.

Differential attenuator 45 comprises a dichroic coupler 451 to separatethe signals at the telecommunication channel wavelengths to a firstoutput and the residual radiation at the wavelength of pumping source 42to a second output. Said first output is connected via an opticalisolator 452 to a first input of a dichroic coupler 454. Said secondoutput is connected via a section of optical fiber to a second input ofdichroic coupler 454. Optical isolator 452 provides an attenuation ofaround 1 dB to the telecommunication signals that transit through it,while the residual pump radiation is not significantly attenuated. Theoptical isolator also blocks the counterpropagating radiation, with theeffect of reducing the amplifier noise. A section of attenuating opticalfiber 454, with predetermined attenuation, can be connected in lieu ofthe optical isolator, or preferably in series with it. Thecharacteristics of this attenuating fiber may be predetermined accordingto the indications contained in the two patent applications cited.

Dichroic coupler 454 combines the residual pump radiation with theattenuated telecommunication signals to active fiber 46, which furtheramplifies the signals.

An optical isolator 461 is placed between the output of the first stageand the input of the second stage.

An output of said isolator terminates in one end of active fiber 47,while the other end is connected to a dichroic coupler 48. Pumpingsource 49 is connected to an input of said dichroic coupler 48 in such away as to feed active fiber 48. An output of dichroic coupler 48 isconnected, by means of an optical isolator 462, to an output 420 of thepreamplifier.

Although the pumping scheme described (copropagating for the first stageand counterpropagating for the second stage) is preferable, otherpumping schemes are equally possible.

The characteristics and type of components of the preamplifier maygenerally be selected according to the previous indications regardingthe power amplifiers described.

In particular, in the case of the preamplifier, the lengths of activefibers 44, 46 and 47 may be advantageously around 7 m, 3 m and 6 m,respectively.

Pumping sources 42 and 49 may be, for example, quantum well lasers withan emission wavelength of λ_(p)=980 nm. The optical emission power isenvisaged at 65 mW for source 42 and 75 mW for source 49.

A preamplifier like the one described gives, for example, output powerof 16 dBm, with a noise figure of 5 dB.

The preamplifier may also be embodied as a single stage amplifier,depending on the gain required and the characteristics of thetelecommunication system in which it is to be used.

Multiplexers 4A and 4B of the system in FIG. 1 are passive opticaldevices, by which the optical signals superposed in a single fiber areseparated on respective optical fibers, depending on their wavelength.

An example of demultiplexer well-suited for use in the present inventionis indicated in FIG. 5A. The figure represents a demultiplexerwell-suited for use in a system with 16 channels, i.e. 16 independentwavelengths, for each path direction. A similar scheme may be employedin cases where the system calls for a different number of channels. Thesignals input to a port 500 are separated by means of a 3 dB splitter,540, to two groups of cascaded wavelength selective splitters 550 and560 (briefly indicated as selective splitters). Each selective splitteris capable of routing to a first output the signals applied to one ofits inputs with wavelengths centered around one of the transmissionchannels employed in the system and of reflecting to a second output thesignals with wavelengths external to that band. Said second output ofeach selective splitter is connected to the input of a successiveselective splitter, so as to form a cascaded connection. In the deviceillustrated in the figure, corresponding to demultiplexer 4B of FIG. 1,group 550 includes selective splitters 501, 503, . . . , 515, selectivearound wavelengths λ₁, λ₃, . . . , λ₁₅ respectively, while group 560comprises selective splitters 517, 519, . . . , 531 selective aroundwavelengths λ₁₇, λ₁₉, . . . , λ₃₁, respectively. The device described iswell-suited for use as demultiplexer 4B in the telecommunication systemof FIG. 1. A similar device, using selective splitters at wavelengthsλ₂, λ₄, . . . , λ₃₂ may be employed to embody demultiplexer 4A of thetelecommunication system in FIG. 1.

The selective splitters may preferably be of the type diagramed indetail in FIG. 5B, having four access optical fibers (input and outputports) designated 591, 592, 593 and 594, respectively, and containing inthe center a selective reflecting component 595 which acts as atransmission bandpass filter and a reflective band-suppression filter,i.e. designed to transmit with low attenuation (e.g. with attenuationlower than 1.5 dB) signals with wavelengths within a predetermined bandand reflecting (with attenuation of the same order of magnitude) signalswith wavelengths outside that band. A signal input to fiber 591 of theselective splitter with wavelength λ_(p) inside the passing band ofcomponent 595, for example, is transmitted to fiber 593 and, similarly,signals at λ_(p) are transmitted from fiber 594 to fiber 592 or,symmetrically, from fiber 593 to fiber 591 and from fiber 592 to fiber594. A signal input to fiber 591 with wavelength λ_(r) outside thatband, on the other hand, is reflected to fiber 594 and similarly signalsat λ_(r) proceed from fiber 592 to fiber 593 and symmetrically fromfiber 594 to fiber 591 and from fiber 593 to fiber 592.

The band of wavelengths, close to a wavelength of minimal transmissionattenuation, which corresponds, in transmission through selectivereflecting component 595, to an attenuation of no more than 0.5 dB inaddition to the minimal attenuation, will be indicated hereinafter as“0.5 dB passband” of selective reflecting component 595 or, byextension, as 0.5 dB passband of the selective splitter.

Likewise, the band of wavelengths, close to a wavelength of minimalreflection attenuation, which corresponds, in reflection throughselective reflecting component 595, to an attenuation of no more than0.5 dB in addition to the minimal attenuation, will be indicatedhereinafter as “0.5 dB reflected band” of selective reflecting component595 or, by extension, as 0.5 dB reflected band of the selectivesplitter.

The selective splitters are selected in a way that, for each of them,the wavelength of one of the communication channels is included in therespective 0.5 dB passband, while the wavelengths of the remainingcommunication channels are included in the respective 0.5 dB reflectedband.

By analogy, the band of wavelengths corresponding in transmissionthrough the selective splitter to an attenuation of no more than 20 dBin addition to the minimal attenuation is indicated as a −20 dB passbandof the selective splitter.

Although described with four access fibers, the selective splitterssuitable for the aforesaid use may have only three access fibers, thefourth (e.g. the one indicated as 594) remaining unused.

Selective splitters of the type indicated and well-suited for use in thepresent invention are marketed, for example, by Optical Corporation ofAmerica, 170 Locke Drive, Marlborough, Mass. (USA).

Selective splitters of the type indicated are now available, e.g., witha 0.5 dB passband of about 0.7 nm and a 20 dB bandwidth of about 2.4 nm.

Selective splitters based on Mach-Zehnder interferometers employingBragg fiber-optic gratings, such as the “Mach-Zehnder based FBG” modelproduced by INNOVATIVE FIBER, are also suitable for use in the presentinvention.

Of possible use in the present invention are also, for example,demultiplexers made, according to the general scheme of FIG. 5A, withgroups of cascaded selective splitters integrated on a single substrate,such as those produced by the aforesaid Optical Corporation of America.

Demultiplexers of the type described may be easily adapted to operatewith a number of channels different from that determined in the systeminstallation phase. It is possible, for example, to add one or moreselective splitters cascaded with the selective splitters alreadypresent, so as to permit the demultiplexing of additional wavelengths.

The number of independent channels transmitted in the system may,through the present invention, be greater than the number of channelsthat can be separated by the available demultiplexers. Thus, forexample, with reference to the example described, a total of 32 channelsare transmitted through the system (16 in each direction) usingdemultiplexers adapted to separate 16 channels.

Multiplexers 3A and 3B of the system in FIG. 1 are passive opticaldevices through which the optical signals at different wavelengths,transmitted on respective optical fibers, are overlapped in a singlefiber. Devices of this type can be made, for example, in the same way asthe demultiplexers just described by interchanging their inputs andoutputs.

A bidirectional multichannel optical amplifier 9 according to thepresent invention, well-suited for use in the telecommunication systemof FIG. 1, will now be described in greater detail with reference toFIG. 6.

Multichannel optical amplifiers 93 and 94 connected between opticalcirculators 91 and 92 in such a way as to amplify the signalspropagating from transmission station 1A to receiving station 2B and,respectively, from transmission station 1B to receiving station 2A, areembodied as wavelength selective optical amplifiers and namely selectiveat the wavelengths λ₁, λ₃, λ₅, . . . , λ₂₉, λ₂₉, λ₃₁ and, respectively,λ₂, λ₄, λ₆, . . . , λ₃₀, λ₃₂.

In a first stage of amplifier 93, a dichroic coupler 62 feeds thecommunication signals coming from an input port 641, connected to anoutput port of optical circulator 91, and the pumping radiation, comingfrom a first optical pumping source 61 connected to dichroic coupler 62,to a first active optical fiber 63, whose output terminates in an inputof a dichroic coupler 671. A first output of dichroic coupler 671 isconnected in input to an optical isolator 672, while a second output ofdichroic coupler 671 is connected to an input of a dichroic coupler 675by means of an optical fiber section, so as to constitute alow-attenuation path for the residual pump radiation downstream ofactive fiber 63.

A comb filter is connected between the output of optical isolator 672and a second input of dichroic coupler 675 by means of low-reflectivityconnectors 673 and 674.

The comb filter has a passband that includes wavelengths λ₁, λ₃, λ₅, . .. , λ₂₉, λ₃₁ of the signals propagating from transmission station 1A toreceiving station 2B. Wavelengths λ₂, λ₄, λ₆, . . . , λ₃₀, λ₃₂ of thesignals propagating in the system in the opposite direction, on theother hand, are external to the passband of said comb filter.

Said comb filter may include, as illustrated in the figure, an opticalcirculator 64 with a selective reflection circuit 65 connected to one ofits intermediate ports. Said circuit 65 comprises serially connectedfilters 601, 603, 605, . . . , 629 and 631, with selective reflection atwavelengths λ₁, λ₃, λ₅, . . . , λ₂₉, λ₃₁ respectively, and is terminatedby a low-reflectivity termination 650.

An output of dichroic coupler 675 terminates in a second active opticalfiber 66, which in turn terminates at in input of an optical isolator676.

Said second active fiber 66 is pumped through the residual pumpradiation from first active fiber 63.

The output of optical isolator 676 is connected to a third activeoptical fiber 67. Active fiber 67 is fed with counterpropagating pumpingradiation through a optical pumping source 69 and a dichroic coupler 68.

An output of dichroic coupler 68 is connected to an output port 642,connected to an input port of optical circulator 92.

In amplifier 93, signals at wavelengths λ₁, λ₃, λ₅, . . . , λ₂₉, λ₃₁input to port 641 are amplified in the first stage of amplification,transmitted by the comb filter through the reflection of each signal byone of the selective reflection filters of circuit 65 and furtheramplified in the second stage of amplification.

Any other signals, or noise, at wavelengths external to the bands ofselective reflection filters 601, 603, . . . , 631, after passagethrough the first amplification stage, are transmitted through circuit65 without being reflected and are eliminated from the circuit throughlow-reflectivity termination 650.

Multichannel amplifier 94 is similar to multichannel amplifier 93. For adescription of the corresponding parts and the general functioning ofamplifier 94, refer therefore to the previous description of amplifier93.

In amplifier 94, the comb filter has a passband that includeswavelengths, λ₂, λ₄, λ₆, . . . , λ₃₀, λ₃₂ of the signals propagatingfrom transmission station 1B to reception station 2A. Wavelengths λ₁,λ₃, λ₅, . . . , λ₂₉, λ₃₁ of signals propagating in the system in theopposite direction are external to the passband of said comb filter.

This comb filter may comprise, as illustrated in the figure, an opticalcirculator 654 with a selective reflection circuit 655 connected to oneof its intermediate ports. This circuit 655 includes serially connectedfilters 602, 604, 606, . . . , 630, 632, with selective reflection atwavelengths λ₂, λ₄, λ₆, . . . , λ₃₀, λ₃₂ respectively. Reflectioncircuit 655 is terminated by a low-reflectivity termination 650.

Optical amplifiers 93 and 94 described are of the two-stage type. Afirst stage of amplification comprises active fiber sections 63, 653 and66, 656. Active fibers 63 and 653 are pumped directly by sources 61 and651 through dichroic couplers 62 and 652. Active fibers 66 and 656,connected downstream from the comb filter, are pumped with residualpumping radiation present at the output of active fibers 63 and 653 bymeans of the low-attenuation path created by connecting togetherdichroic couplers 671, 675 and 681, 685, respectively.

The signal attenuation by optical isolator 672, optical circulator 64and selective reflection circuit 65, connected along the optical signalpath in the section between dichroic couplers 671,675 and 681, 685,respectively, and the reduced attenuation of the residual pump radiationcompress the signal dynamics in the first amplifier stage, according tothe mechanism previously illustrated with reference to differentialattenuator 45 of the device in FIG. 4.

A second stage of amplification comprises active fiber sections 67 and657, which are pumped by pumping sources 68 and 658 through dichroiccouplers 69 and 659.

The second stage, operating in saturation, further compresses the signaldynamics.

The length of active fiber 67, 657 of the second stage is to advantagearound ⅔ the total length of the active fiber of the first stage (fiber63, 66).

The length of active fiber 66,656, connected downstream from the combfilter, is to advantage around half the length of active fiber 63, 653,connected upstream from the comb filter.

If the active fibers used are of the type previously indicated withreference to the power amplifier in FIG. 3, for example, the lengths ofactive fibers 63 and 653, 66 and 656, 67 and 657 may be around 7 m, 3 mand 6 m, respectively.

Active fiber 66, 656 may be used to good advantage, according to thedescription, to compensate at least in part for the signal attenuationby the comb filter.

Said active fiber 66, 656 may be omitted, however, particularly if theattenuation of the comb filter is sufficiently low. If fiber 66, 656 isnot present in the amplifier, it is also possible to omit thelow-attenuation path for the pumping radiation, comprising dichroiccoupler 671, 675 and 681, 685, respectively, and the respectiveconnecting optical fibers. In this case, active fiber 63, 653 isconnected directly to the input of optical isolator 672, 682 andconnector 674, 684 directly connects the input port of opticalcirculator 64, 654 and the input of optical isolator 676, 686.

Optical amplifiers 93, 94, depending on the required gain and thecharacteristics of the telecommunication system in which it is to beused, may also be embodied as single-stage amplifiers. It is possible,for example, with reference to device 93 of FIG. 6, to omit the secondstage comprising active fiber 67, dichroic coupler 68 and pump source69. This simpler configuration may be sufficient to cover shortersections of optical line.

Although the embodiment described with reference to FIG. 6 ispreferable, particularly in terms of noise figure and output power,another alternative would be to connect the comb filter downstream orupstream from the power amplifier, respectively.

A bidirectional multichannel optical amplifier 9 may be realized byusing, where no otherwise specified, components similar to thosepreviously described with reference to the devices in FIGS. 3 and 4.

Pump sources 61, 69, 651, 659, for example, may be quantum well laserswith an emission wavelength λ_(p)=980 nm. The optical emission powerenvisaged is around 90 mW for each source.

Optical connectors 676, 674, 683, 684, for example, are connectors withreflectivity of less than −40 dB. Connectors of this type are produced,for example, by the aforesaid firm SEIKOH GIKEN.

Selective reflection filters well-suited for use in the presentinvention, for example, are distributed Bragg reflection opticalwaveguide filters. They reflect the radiation within a narrow wavelengthband and transmit the radiation outside said band. They consist of aportion of optical waveguide, e.g. optical fiber, along which an opticalparameter, e.g. the refractive index, has a periodic variation. If thereflected portions of the signal at each change of index are mutually inphase, constructive interference occurs and the incident signal isreflected. The condition of constructive interference, corresponding tomaximum reflection, is expressed by the relationship 2·I=λ_(s)/n, whereI indicates the pitch of the grating formed by the variations in theindex of refraction, λ_(s) the wavelength of the incident radiation andn the refractive index of the waveguide core. The phenomenon describedis indicated in the literature as distributed Bragg reflection.

A periodic variation of the refractive index may be obtained by knowntechniques, e.g. by exposing a portion of optical fiber, deprived of itsprotective coating, to the interference fringes formed by an intense UVbeam (like that generated by an excimer laser, a frequency-doubled argonlaser or a frequency-quadrupled Nd:YAG laser) made to self-interfere bya suitable interferometric system, e.g. by means of a silica phase mask,as described in U.S. Pat. No. 5,351,321. The fiber, and particularly thecore, are thus exposed to UV radiation of an intensity varyingperiodically along the optical axis. In the areas of the core reached bythe UV radiation the Ge—O bonds are partially broken causing a permanentchange in the refraction index.

The central wavelength of the reflected band can be determined at willby selecting a grating pitch that results in the constructiveinterference relationship.

With this technique it is possible to obtain filters with a −3 dBreflected wavelength band of only 0.20-0.3 nm, reflectivity at thecenter of the band almost up to 100%, a central wavelength of thereflected band that can be determined in the construction phase within+/−0.1 nm and a temperature variation of the central wavelength of theband not greater than 0.02 nm/° C.

Optical distributed Bragg reflection filters with a broader reflectionband can be realized by gradually chirping the grating pitch along itsextension between two values, corresponding to the wavelengths thatdelimit the desired reflection band.

Optical fiber distributed Bragg reflection filters with chirped gratingare known, for example, from the article by P. C. Hill et al. publishedin Electronic Letters, vol. 30 no. 14, Jul. 7, 1994, pages 1172-74.

The gradual variation of the grating pitch, in a distributed Braggreflection filter, may also be employed to realize filters capable ofcompensating for the delay (or advance) of some chromatic components ofan optical signal with respect to others. For this reason, components ofa signal with different wavelengths must be reflected by differentportions of the same grating, displaced on an optical path so as tocompensate for said delay or said advance.

Chromatic dispersion, i.e. the delay (or advance) per wavelength unit ofa grating having a pitch that may vary between two extreme values,depends not only on the width of the reflected band but also on thelength of the grating or, in greater detail, on a quantity equal totwice the length of the grating multiplied by the effective index ofrefraction of the means in which it is embodied. This quantitycorresponds to the difference between the optical paths of the signalchromatic components which are reflected close to the two extremes ofthe grating.

The use of distributed Bragg reflection filters for compensatingchromatic dispersion is known, for example, from the aforementionedarticle by F. Ouellette published in Optics Letters or from U.S. Pat.No. 4,953,939.

To compensate for the chromatic dispersion at the communication signalwavelengths, it is possible to use as selective reflection filters 601,603, . . . , 631 and 602, 604, . . . , 632 optical fiber distributedBragg reflection filters with chirped grating.

In this case, each of the filters will be realized with a centralwavelength and passband width suitable to reflect radiationcorresponding to one of the communication channels, and with dispersioncharacteristics that compensate for the chromatic dispersion of thecorresponding communication channel.

Depending on the conditions under which the device is used, the filtersmay be realized in such a way as to provide the reflected communicationsignal with a chromatic dispersion equal in absolute value, and ofopposite sign, to that (estimated or measured) accumulated by the signalthrough the fiber sections it has traveled, or such as to overcompensatefor the dispersion accumulated by the signal, so that the dispersion isnullified at a successive point on the optical signal path, including anadditional section of optical fiber.

If the amplifier is used under conditions characterized by significantvariations in temperature, it may be advisable to thermally stabilizefiber optic filters 601, 603, . . . , 631 and 602, 604, . . . , 632.

The optical output power of an optical amplifier 93 or 94 as describedis, in an example, about 16 dBm under operating conditions, withcirculators 91, 92 connected to the two extremes and with optical inputpower of −10 dBm. The noise figure is around 5 dB.

The Applicant has observed that optical circulators 91 and 92 permitradiation to enter and exit in only one direction for each of opticalamplifiers 93, 94 and precisely only the radiation propagating fromtransmission station 1A to receiving station 2B for amplifier 93 andonly the radiation propagating from transmission station 1B to receivingstation 2A for amplifier 94.

Optical circulators 91 and 92 therefore act as unidirectional componentsplaced at the input and output of the two stages of optical amplifiers93 and 94 and reduce the noise, particularly that due tocounterpropagating spontaneous emission, Rayleigh and Brillouinscattering and their respective reflections along the communicationline.

In addition to permitting the bidirectional amplification of thesignals, the bidirectional amplifier described attenuates thepropagating amplified spontaneous emission (ASE) along with the signals.In amplifiers 93 and 94, the ASE components coming from inputs 641 and643 and those generated in active fibers 63 and 653 are removed by therespective comb filters and do not propagate to active fibers 66 and656.

The Applicant has determined that bidirectional amplifier 9 functionsstably without oscillations and with negligible interferometric noise.This is thought to derive from the fact that the arrangement of thesignal wavelengths, along with the spectral characteristics of the combfilters, prevents the creation of possible feedback rings, includingamplifiers 93 and 94, which might be formed in the presence ofback-reflections along the optical fibers of line 8, e.g. by connectorsof optical circulators 91 and 92 with said optical fiber of line 8.

An optical amplifier according to the present invention is well-suitedfor use not only along communication lines configured to have lowreflections (e.g. employing low-reflection optical connectors and welds)but also along optical communication lines already installed and in thepresence of components with non-negligible residual reflectivity,particularly if they are used along fiber-optic transmission lines inwhich the amplifier is connected to the line fibers by means of opticalconnectors, which may be of the type that transmit most of the power ofthe signals transiting through them, and thus ensure the opticalcontinuity of said signals, but which under some conditions reflect backa small portion of them (e.g. in case of an imperfect clamping caused byincorrect positioning of the two fiber ends inside them).

Nonetheless, to obtain a high signal/noise ratio in the transmissionalong the telecommunication system, such as to permit transmission atvelocities greater than or equal to 2.5 Gb/s, the optical connectionslinking an optical amplifier 9 and optical communication line 8 havepreferably a reflectivity of less than −31 dB, more preferably less than40 dB. Furthermore, to facilitate the operations of line installationand maintenance, these optical connections should be realized withoptical connectors.

The Applicant has determined that an optical amplifier of the typedescribed minimizes the gain tilt, a phenomenon caused by thecharacteristics of the doped fiber and, in particular, by the relativelevel of amplified spontaneous emission (ASE) and the signals along thecommunication line and in the amplifiers cascaded along it, whichconsists of a variation in gain with the wavelength and results indifferent gains for the various channels.

Exploiting the small residual attenuation of the selective reflectionfilters in the band transmitted (about 0.1 dB, for example, for passagein each direction through each Bragg grating selective reflectionfilter), it is possible to arrange said filters, in the selectivereflection circuit that is part of the comb filter, in an order suchthat it compensates for the differences in gain.

In greater detail, the channels subject to less gain can be attenuatedto a lesser degree by connecting the selective reflection filtersrelated to those channels in proximity to the end of the selectivereflection circuit that is connected to optical circulator 64 (thesignals are reflected after passing through a limited number ofselective reflection filters, thus with less attenuation), and thechannels subject to greater gain can be attenuated to a greater degreeby connecting the respective selective reflection filters in proximityto the opposite end of the selective reflection circuit.

Should it be necessary to compensate for the gain tilt to a greaterextent than permitted by the selective attenuation provided by thefilters, sections of optical fiber with calibrated attenuation may beconnected between the selective reflection filters.

To compensate for a predetermined difference in gain, in output to anamplifier, between signals of different wavelengths, the difference inattenuation of the two signals in the comb filter must generally begreater, in absolute value, than said predetermined difference in outputgain.

In the configuration described with reference to FIG. 6, the distancesbetween the filters connected along the selective reflection circuitincrease as the wavelength increases, so that the attenuation of eachchannel is attenuated by 0.2 dB more ( due to the double passage) thanthe adjacent channel at a lower wavelength.

In one example, the Applicant evaluated the functioning of abidirectional multichannel telecommunication system like the onedescribed, in a configuration including five sections of optical fiber8, each with maximum total attenuation of 26 dB (including attenuationat the optical splices), connected by four bidirectional amplifiers 9,each of the type described.

The Applicant has determined that this communication system permits thesimultaneous transmission of 16 channels in each direction ofpropagation at a bit rate of 2.5 Gb/s, with a minimum signalunoise ratioof 13 dB (measured on an 0.5 nm band).

In a second example, the Applicant evaluated the functioning of abidirectional multichannel communication system like the one describedbut configured to operate with 8 wavelengths in each direction ofpropagation, with the wavelengths of the signals propagating in onedirection staggered with respect to those of the signals propagating inthe opposite direction. The configuration considered includes fivesection of fiber-optic line 8, each with a maximum total attenuation of28 dB (including the attenuation of the optical junctions), connected byfour bidirectional amplifiers 9, each of the type described.

The Applicant determined that said communication system permits thesimultaneous transmission of 8 channels in each direction of propagationat a bit rate of 2.5 Gb/s, with a minimum signalvnoise ratio of 13 dB(measured on a 0.5 nm band).

In another example, regarding a communication system similar to the onein the second example but where the total maximum attenuation of eachfiber-optic line section is 23 dB (including the attenuation of theoptical splices), and in which the four bidirectional amplifiers includechromatic dispersion compensation means of the type indicated earlier,the Applicant determined that it is possible to transmit 8 channelssimultaneously in each direction of propagation at a bit rate of 10Gb/s, with a minimum signal/noise ratio of 18 dB (measured on a 0.5 nmband).

It is known that an optical communication system may assume thestructure of an optical network connecting a number of stations to eachother. Optical network is generally intended here to mean a set offiber-optic transmission lines and their respective stations ofinterconnection, also known as interchange nodes. In the interchangenodes the optical signals can be routed from one of the transmissionlines linked to the node to one or more of the other transmission lineslinked to the node. Nodes for adding and dropping optical signals to orfrom the network may be positioned either along the transmission linesor at the interchange nodes. Some of the transmission lines in thisoptical network, in particular, may have a ring structure.

A particular example of optical network with nodes for adding ordropping signals is that of a WDM communication system comprising afiber-optic line extended between transmission and receiving stationsand intermediate stations for adding/dropping signals placed along theline. The signals at various wavelengths emitted by a transmissionstation propagate along an optical fiber, possibly through amplifiers,e.g. of the active optical fiber type, up to an intermediate signaladdition/dropping station, which may be configured in such a way thatthe radiation to some of the signal wavelengths is dropped from thecommunication line and routed to specific receivers (which, for example,convert the signals into electrical form), while at the same timeradiation to one or more of the same wavelengths, generally modulated bytransmission signals (e.g. in electrical form) present at the input ofthe intermediate station, is inserted into the communication linedownstream from the dropping point. The optical radiation output fromthe intermediate station is transmitted along an additional section ofoptical fiber, and possibly through additional amplifiers andintermediate stations for adding/dropping optical signals, until itreaches a receiving station.

Each wavelength constitutes an independent communication channel. Theoptical telecommunication system may be configured in such a way that ittransmits optical signals separately between pairs of stations includedbetween the terminal stations and the stations placed along the line. Itis also possible to transmit independent signals with the samewavelength along lines without common sections.

In this communication line it is possible to add or drop signals atvarious points (nodes) along the line at some of the communicationwavelengths, so that they travel only a portion of the line extension.

A scheme of a multichannel optical amplifier comprising a device of thefirst type for adding/dropping optical signals will now be describedwith reference to FIG. 7.

The figure represents an optical amplifier 93′ suitable for use in atelecommunication system of the type described with reference to FIG. 1,in lieu of one or more optical amplifiers 93 of said system. In theexample indicated in FIG. 7, amplifier 93′ is suitable for amplifyingoptical signals at wavelengths λ₅, λ₇, . . . , λ₃₁, propagating fromtransmission station 1A to receiving station 2B, for dropping opticalsignals at wavelengths λ₁, λ₃, from the communication line and foradding new signals at wavelengths λ₁, λ₃, to the same line. The schemein FIG. 7 may be modified, applying known techniques, in such a way asto adapt it to the wavelengths to be amplified/dropped/added in eachcase of interest. It is possible, for example, to build an amplifier,not represented in the figure, to amplify optical signals at wavelengthsλ₆, λ₈, . . . , λ₃₂, propagating from transmission station 1B toreceiving; station 2A, to drop signals at wavelengths λ₂, λ₄, and to addnew signals at wavelengths λ₂, λ₄, to the same line.

Optical amplifier 93′ includes one or more amplification stages betweenan input port 741 and an output port 742. The example indicates twoamplification stages 71 and 72 that may, for example, be analogous tothe amplification stages previous described with reference to amplifier93 of FIG. 6.

A comb filter is connected in series, between input 741 and output 742,to a signal adding/dropping device.

The position of said comb filter may be determined as previouslyindicated in relation to the amplifier of FIG. 6.

According to the example illustrated in FIG. 7, a comb filter comprisesan optical circulator 64′, at an intermediate port of which is connecteda first end of a selective reflection circuit 65′, including filters605, 607, . . . , 631, with selective selection at wavelengths λ₅, λ₇, .. . , λ₃₁ , and for the rest equal to selection filter 65 of FIG. 6.

A second end of said selective reflection circuit 65′ is connected to anintermediate port of an optical circulator 73. An output port of opticalcirculator 73 is connected to a selective splitter 701, with passbandcentered around wavelength λ₁ and wide enough to exclude the adjacentwavelengths propagating along the communication line in the samedirection of signal propagation as wavelength λ₁. Said selectivesplitter may be, for example, of the type described with reference toFIG. 5B. Connected in series to selective splitter 701 is a secondselective splitter 703, with a passband of similar width but centeredaround wavelength λ₃.

The device formed by cascading selective splitters 701, 703, accordingto what was indicated with reference to FIG. 5A, creates a demultiplexerat wavelengths λ₁, λ₃. The signals at said wavelengths are madeavailable, at outputs 75, 76 respectively, to an outside user,consisting, for example, of a pair of optical receivers.

A device symmetric to the one described, comprising selective splitters751, 753, similar respectively to selective splitters 701, 703, forms aninput multiplexer, connected to an input port of optical circulator 73,which can send to said second end of selective reflection circuit 65′the respective optical signals at wavelengths λ₁, λ₃ present at inputs77, 78 of selective splitters 751, 753.

In amplifier 93′, signals at wavelengths λ₅, λ₇, . . . , λ₃₁, areamplified in amplification stage 71, transmitted by the comb filterconnected between the two stages through the reflection of each signalby one of the selective reflection filters of circuit 65′, furtheramplified in amplification stage 72 and sent to output 742. Signals atwavelengths λ₁, λ₃, on the other hand, after being amplified throughamplification stage 71, pass through selective reflection circuit 65′and are dropped to respective outputs 75, 76. Other signals atwavelengths λ₁, λ₃, present at inputs 77, 78 of selective splitters 751,753, pass through selective reflection circuit 65′ and are combined,through optical circulator 64′, with the signals at wavelengths λ₅, λ₇,. . . , λ₃₁, and are then amplified with them in amplification stage 72and then sent to output 742.

The scheme of a multichannel optical amplifier comprising a device of asecond type for adding/dropping optical signals will now be describedwith reference to FIG. 8.

The figure shows an optical amplifier 931 suitable for use in atelecommunication system of the type described with reference to FIG. 1,in lieu of one or more optical amplifiers 93 of said system. In theexample shown in FIG. 8, amplifier 93″ is suitable for amplifyingoptical signals at the 16 wavelengths λ₁,λ₃, . . . , λ₂₉, λ₃₁,propagating from transmission station 1A to receiving station 2B and,among said signals, dropping/adding fromrinto the optical communicationline one or more signals at the 8 wavelengths , λ₁, λ₇, . . . , λ₂₇,λ₃₁, according to a scheme that may be selected according torequirements, by means of appropriate control signals.

By modifications according to known techniques of the scheme that willbe described in the following, an optical amplifier can be made,symmetrical to the previous one, suitable for amplifying optical signalsat the 16 wavelengths λ₂,λ₄, . . . , λ₃₀, λ₃₂, propagating fromtransmission station 1B to receiving station 2A and, among said signals,dropping/adding from/into the optical communication line one or moresignals at the 8 wavelengths λ₄, λ₈, . . . , λ_(28, λ) ₃₂, according toa scheme that may be selected according to requirements, by means ofappropriate control signals.

Optical amplifier 93″ comprises one or more amplification stages betweenan input port 841 and an output port 842. The example indicates twoamplification stages 81, 82 which may be, for example, similar to theamplification stages described earlier with reference to amplifier 93 ofFIG. 6.

A comb filter, cascaded to a signal add/drop device, is connected at anintermediate position between input port 841 and output port 842.

The position of this filter may be determined based on what wasindicated earlier regarding the amplifier of FIG. 6.

According to the example illustrated in FIG. 8, a comb filter comprisesan optical circulator 64″, at an intermediate port of which a first endof a first selective reflection circuit 84 is connected, comprising 8filters 601, 605, . . . , 629, with selective reflection at wavelengthsλ₁, λ₅, . . . , λ₂₉, respectively, and for the rest similar to selectivereflection circuit 65 of FIG. 6.

Signals at wavelengths λ₁, λ₅, . . . , λ₂₉, are reflected by one of theselective reflection filters to said intermediate port of opticalcirculator 64″ and by it to the second stage 82 of the amplifier andthen to output 842.

Signals at wavelengths λ₃, λ₇, . . . , λ₃₁, on the other hand, are notreflected by said selective reflection circuit 84 and are transmitted toa second end of said circuit 84, which is connected to a first port ofan optical circulator 85, from there to a second port of opticalcirculator 85, following in a sequence said first port, and then to ademultiplexer 86, connected to said second port. Said demultiplexer 86(which may be embodied, for example, by cascaded wavelength selectivesplitters, as described with reference to FIG. 5A) separates each signalat wavelengths λ₃, λ₇, . . . , λ₃₁, to a different optical path. Theoutput of said optical paths is connected to an input of opticalswitches 871, 872, . . . , 878.

In addition to a first optical input, connected to demultiplexer 86,said optical switches also have a second optical input, which may beconnected to transmitters 851, . . . , 858, suitable for generatingsignals at wavelengths λ₃, λ₇, . . . , λ₃₁, and two optical outputs. Thefirst of said optical outputs is connected to an optical multiplexer 87,suitable for multiplexing the signals at different wavelengths into asingle output, while the second of said optical outputs is connected toan optical receiver.

By means of appropriate control signals, fed to each of said switches,it is possible to modify their transmission state in one of thefollowing two ways: bar mode, corresponding to the direct connection ofsaid first input with said first output, and cross mode, correspondingto the connection of said first input with said second output and,respectively, of said second input with said first output.

The optical switches may be, for example, model SR2:2 of JDS FITEL Inc.,570 Heston Drive, Nepean, Ontario (Canada).

The signals switched to said second outputs are made available toexternal users consisting, for example, of optical receivers connectedto said second outputs.

The signals coming from said first outputs, on the other hand, are sentto a multiplexer 87, multiplexed by it onto a single output and sent,possibly through an optical amplifier 88, to a third port of saidoptical circulator 85.

Optical amplifier 88, of a known type, is suitable for compensating theattenuation of the signals at wavelengths λ₃, λ₇, . . . , λ₃₁, in theportion of the optical circuit between the first port of opticalcirculator 85 and the optical amplifier itself. At the same time,optical amplifier 88 amplifies the signals input into the system throughone of optical switches 871, 872, . . . , 878, bringing said signals toa power level comparable to that of the other signals at the differentwavelengths.

The signals input to the third port of optical circulator 85 are thentransmitted to a fourth port of the same optical circulator, to which isconnected a first end of a second selective reflection circuit 89,including filters 603, 607, . . . , 631, with selective reflection atwavelengths λ₃, λ₇, . . . , λ₃₁, respectively. The signals at saidwavelengths are reflected to the fourth port of the optical circulator,from it to the first port of same and, through selective reflectioncircuit 84 and optical circulator 64″, are combined with the remainingsignals. The signals with wavelengths external to the reflection band offilters 603, 607, . . . , 631, including noise at intermediatewavelengths, are eliminated from the optical circuit throughlow-reflection termination 650, connected to a second end of selectivereflection circuit 89.

A multichannel amplifier 93″ of the described type makes it possible toamplify a certain number of wavelengths (λ₁, λ₅, . . . , λ₂₉) and can beconfigured to drop and/or add to the system one or more signals atdifferent wavelengths (λ₃, λ₇, . . . , λ₃₁).

The Applicant observes the described amplifier 93″ may include anoptical demultiplexer 86 and an optical multiplexer 87 with a relativelylow wavelength resolution. The demultiplexer and multiplexer, namely,need only be suitable for separating signals different in wavelengthfrom twice the minimum wavelength distance between signals propagatingin the same direction in the optical communication system.

What is claimed is:
 1. Optical amplifier comprising: a first input forinputting first optical signals propagating in a first direction havinga first series of mutually distinct wavelengths; a first output foroutputting second optical signals propagating in a first directionhaving said first series of mutually distinct wavelengths; a firstamplifier for amplifying said first optical signals; and a first opticalcirculator having an input port coupled to said first amplifier, anintermediate port coupled to a first end of a first selective reflectioncircuit, an output port coupled to said first output; and furthercomprising a second optical circulator having an intermediate portcoupled to a second end of said first selective reflection circuit; anoutput port coupled to a first selective splitter, with passbandcentered around a first predetermined wavelength of said first seriesand wide enough to exclude the adjacent wavelengths of said firstseries; and an input port coupled to a second selective splitter, withpassband centered around said first predetermined wavelength of saidfirst series and wide enough to exclude the adjacent wavelengths of saidfirst series.
 2. Optical amplifier according to claim 1, furthercomprising a third selective splitter coupled in series to said firstselective splitter, with passband centered around a second predeterminedwavelength of said first series and wide enough to exclude the adjacentwavelenghts of said first series.
 3. Optical amplifier according toclaim 1, further comprising a fourth selective splitter coupled inseries to said second selective splitter, with passband centered arounda second predetermined wavelength of said first series and wide enoughto exclude the adjacent wavelengths of said first series.
 4. Opticalamplifier according to claim 1, further comprising a second amplifiercoupled to said first output for amplifying said second optical signals.5. Optical amplifier according to claim 1, further comprising: a secondinput for inputting third optical signals propagating in a seconddirection, opposite said first direction having a second series ofmutually distinct wavelengths; a second output for outputting fourthoptical signals propagating in a second direction, opposite said firstdirection having said second series of mutually distinct wavelengths;means for separating said first optical signals from said fourth opticalsignals; a third amplifier for amplifying said third signals; and meansfor separating said second and said third signals; wherein said firstseries of mutually distinct wavelengths and said second series ofmutually distinct wavelengths are mutually staggered.
 6. Opticalamplifier according to claim 5, further comprising: a third opticalcirculator having an input port coupled to said third amplifier, anintermediate port coupled to a first end of a second selectivereflection circuit, and an output port coupled to said second output;and a fourth optical circulator having an intermediate port coupled to asecond end of said second selective reflection circuit; an output portcoupled to a fifth selective splitter, with passband centered around afirst predetermined wavelength of said second series and wide enough toexclude the adjacent wavelengths of said second series; and an inputport coupled to a sixth selective splitter, with passband centeredaround said first predetermined wavelength of said second series andwide enough to exclude the adjacent wavelengths of said second series.7. Optical amplifier according to claim 5, wherein said means forseparating include respective optical circulators.
 8. Optical amplifieraccording to claim 1 or 6, wherein said selective reflection circuitcomprises Bragg grating filters.
 9. Optical amplifier according to claim1, wherein said selective reflection circuit reflects the wavelengths offirst series of mutually distinct wavelengths except said firstpredetermined wavelength.
 10. Multichannel optical telecommunicationsystem for transmitting optical signals comprising: a fiber-optic linehaving a first and a second end; a transmission station inputting, atsaid first end of said line, first optical signals propagating in afirst direction and having a first series of mutually distinctwavelengths; a first demultiplexer at said second end of said line; andan amplifier optically connected along said fiber-optic line, having afirst input for inputting said first optical signals, for amplifyingsaid first optical signals on said first path; characterized by the factthat said amplifier for amplifying said first optical signals is coupledto a first end of a comb filter, and a second end of said comb filter iscoupled to a first selective splitter, with passband centered around afirst predetermined wavelength of said first series and wide enough toexclude the adjacent wavelengths of said first series, and to a secondselective splitter, with passband centered around said firstpredetermined wavelength of said first series and wide enough to excludethe adjacent wavelengths of said first series.